I loaded the file in Adobe Acrobat thinking that it did not have extended features for Adobe Reader. I could see that it was not an Adobe form and that it only had a rectangle for the intended Signing Certificate field.
I loaded the PDF into Adobe LifeCycle Designer and chose the Signature field and dragged it over the rectangle. I then expanded the field to cover the rectangle you had in place then saved the PDF.
I then loaded it in Adobe Acrobat and chose Advanced --> Extend features in Adobe Reader then saved the form. If you don't extend features in Adobe Reader then only Adobe Acrobat can save data to the form.
I'm looking for the capability to digital sign a MS Word or Powerpoint document Adobe LifeCycle Designer is beyond the use and scope of a typical home user.